Epidemiology studies indicate that ultraviolet (UV) radiation causes skin damage and is a major environmental agent in the causation of skin cancer. While it has been established that both the constitutive and inducible forms of cyclooxygenase (COX-1 and COX-2, respectively) play important roles in chemical induced skin tumors, the contribution of these two enzymes to UV light-induced skin tumors has not been fully assessed. [unreadable] [unreadable] The cyclooxygenases, COX-1 and COX-2, are involved in cutaneous responses to both acute and chronic UV exposure. In the present studies, wild-type (WT), COX-1-/- and COX-2-/- mice were used to determine the influence of the individual isoform on mouse skin responses to acute UVB treatment. Immunohistochemistry and Western analysis indicated that COX-2, and not COX-1, was induced by UVB (2.5 or 5.0 kJ/m2), but that COX-1 remained the major source of prostaglandin E2 production. UVB exposure significantly increased epidermal apoptosis in all genotypes compared to untreated mice. However, while the number of apoptotic cells in WT and COX-1-/- mice were about equal, the number of apoptotic cells was 2.5-fold greater in COX-2-/- mice. Apoptosis in WT and COX-2-/- mice peaked at 24 h post-exposure. The increased apoptosis and reduced proliferation in COX-2-/- mice resulted in about a 50% decrease in epidermal thickness at 24-48 h post-exposure compared to about a 50% increase in epidermal thickness in WT mice. [unreadable] [unreadable] To better understand the contribution of COX-1 and COX-2 to UV carcinogenesis, we transferred the null allele for each isoform onto the SKH-1 hairless strain of mouse. Due to low viability of COX-2 on SKH-1 mice, heterozygous mice were used in the UV induced carcinogenesis experiments. While the lack of one allele of COX-1 had no effect on tumor outcome, the lack of one allele of COX-2 resulted in a 50-65% reduction in tumor multiplicity and a marked decrease in tumor size. The lack of one allele of either COX-1 or COX-2 reduced prostaglandin (PG) E2 levels in response to a single UV treatment. The proliferative response to UV was significantly reduced in COX-2, but not COX-1, heterozygous mice. UV-induced apoptosis, however, was greater in COX-2 heterozygous mice. Collectively, these results clearly establish the requirement for COX-2 in the development of skin tumors.[unreadable] [unreadable] Overall, the data indicate that COX-2 induction initially protects against the acute sunburn effects of UVB, but that continuous induction of COX-2 may contribute to skin cancer in chronic UVB exposure.